fix: route forward skip-layer edges around intermediate boxes

[cristipufu]

Summary

• Forward edges that skip layers (e.g. parse_agent_output → __end__ skipping call_tool and aggregate_tool_results) were drawing straight vertical lines through intermediate boxes, corrupting node text and overlapping labels
• Add right-side corridor routing for colliding forward edges, matching the existing backward edge corridor pattern
• Uses direct array access for drawing (no performance regression — ~5.2ms total across 9 samples)
• Accounts for label width in corridor margin calculation so labels aren't truncated at canvas edge

Before

Forward skip-layer edges cut straight through intermediate node boxes, overwriting their text.

After

Changes

• src/graphtty/renderer.py — Add _forward_skip_corridors() detection + _draw_forward_corridor() drawing, merge into corridor map, pass route_x/all_boxes to forward edge dispatch
• tests/test_renderer.py — 3 new tests (skip edge collision, skip edge labels, function-agent sample)
• samples/function-agent/ — New sample graph with skip-layer edges
• scripts/generate_screenshots.py — Add function-agent screenshot generation
• screenshots/function-agent.png — New screenshot
• README.md — Updated benchmark table (8 → 9 samples)

Test plan

• All 82 tests pass (uv run pytest tests/ -v)
• Type checks pass (uv run ty check)
• Linting passes (uv run ruff format . && uv run ruff check --fix .)
• Benchmark shows no regression (~5.2ms total across 9 samples)
• Visual verification of function-agent render

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